This invention relates to a process of preparing a solution which contains cesium and rubidium salts and has a density between 1.6 and 3.3 g/cm.sup.3. This process includes hydrothermal digestion of uncalcined pollucite and/or calcined lepidolite with an aqueous solution of Ca(OH).sub.2 at a digestion temperature of 200 to 280.degree. C. and under a pressure from 15 to 65 bars and at a suspension density between 8 and 18% by weight for 0.5 to 3 hours, separation from the insoluble solids, optional removal of calcium ions and lithium ions by an aeration with carbon dioxide and removal of the precipitated carbonates from the digestion filtrate and formation of the salts of cesium and rubidium by an addition of an acid or an acid anhydride to adjust the pH to at least 6. The density of the solution of cesium and rubidium salts is increased by an evaporation performed after the digestion, after the removal of the precipitated carbonates and/or after the addition of the acid or the acid anhydride.
The periodical "Tsvetnye metally--The Soviet Journal of Non-Ferrous Metals", Volume II, No. 5, pages 57 to 59 (1961) discloses a method of preparing cesium carbonate by a hydrothermal digestion of a pollucite-spodumene concentrate. The calcined minerals are subjected to a hydrothermal digestion with an aqueous solution of Ca(OH).sub.2 at 220.degree. C. and under a pressure of 20 atm for four hours. Optimum digestion conditions are achieved with 3 moles Ca(OH).sub.2 per mole SiO.sub.2. 88.3 percent by weight of the cesium contained in the mineral can be recovered and the alumo-cesium-alum can be recrystallized to prepare a cesium salt having a purity &gt;99%. The Chemical Abstracts Reference 79/4949v (1973) discloses the conversion of Cs.sub.2 CO.sub.3 to CsHCO.sub.2 in a process in which the carbonate is reacted with formic acid in water.
Besides, the unpublished German Patent Application P 42 37 954.7 describes a process of preparing solutions which contain cesium salts and rubidium salts and have a density from 1.6 to 3.3 g/cm.sup.3 by a hydrothermal digestion of minerals which contain cesium and rubidium. That process has the process features mentioned first hereinbefore.